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On 6/26/19 13:35, Gary E. Miller wrote:<br>
<blockquote type="cite"
cite="mid:20190626133530.0fbab74f@rellim.com">
<pre class="moz-quote-pre" wrap="">Yo Paul!
On Wed, 26 Jun 2019 13:14:36 -0700
Paul Theodoropoulos <a class="moz-txt-link-rfc2396E" href="mailto:paul@anastrophe.com" moz-do-not-send="true"><paul@anastrophe.com></a> wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">I think maybe I'm hitting a bug or I'm just not understanding
(usually the latter), as it doesn't seem possible:
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">No one totally understands this.</pre>
</blockquote>
There is some comfort in that.<br>
<blockquote type="cite"
cite="mid:20190626133530.0fbab74f@rellim.com">
<blockquote type="cite">
<pre class="moz-quote-pre" wrap=""><a class="moz-txt-link-freetext" href="https://i.imgur.com/LorPR2F.png" moz-do-not-send="true">https://i.imgur.com/LorPR2F.png</a>
jitter fluctuating between 400 parts per trillion and 800 ppt?? From
a Raspberry Pi running at 1.4Ghz?
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">Here is mine:
<a class="moz-txt-link-freetext" href="https://pi3.rellim.com/day/#local_rms_frequency_jitter" moz-do-not-send="true">https://pi3.rellim.com/day/#local_rms_frequency_jitter</a>
My Standard Deviation is 48.504 ppb (10e-9)
Your Standard Deviation is 82.8 10e-12)
So about 500x better then mine.
But my numbers are over 24 hours, not one. So the diurnal ambient changes
are in mine, not yours.
I get a lot better when I put the RasPi in a temp chamber and control
the CPU temp. But not 500x better.</pre>
</blockquote>
Yeah. 500x seems impossible to me - I mean, the processor clock on
the RPI3 is 1.4 billion cycles per second - so I don't see how you
could quantify an interval a small fraction of that cycle. But I've
never claimed that math was my strong suit!<br>
<blockquote type="cite"
cite="mid:20190626133530.0fbab74f@rellim.com">
<pre class="moz-quote-pre" wrap="">That said, I switched to min/maxpoll 1 for PPS, and saw a
corresponding change - so Time jitter appears to be inversely
correlated to Frequency jitter - less frequency jitter, more time
jitter, and vice-versa. This is all fascinating.
</pre>
<pre class="moz-quote-pre" wrap="">This sort of relates to a current discussion on time-nuts. They
are discussing the relative merits of a PLL vs an FLL. Similar effect
here.
The shorter poll keeps the time closer to the PPS, but magnifies the
jitter.
"And if you go chasing rabbits, And you know you're going to fall..."
</pre>
</blockquote>
<br>
Yup - here's a shot showing the effects more clearly - at
min/maxpoll 0, 2, then 1. 1 sure seems like the best compromise, as
the time jitter returned to roughly similar values as the original 0
- though a longer window into this will obviously be more
informative.<br>
<br>
<a class="moz-txt-link-freetext"
href="https://i.imgur.com/8DilX0W.png" moz-do-not-send="true">https://i.imgur.com/8DilX0W.png</a><br>
<br>
<br>
<a class="moz-txt-link-freetext"
href="https://i.imgur.com/ln7FGoA.png" moz-do-not-send="true">https://i.imgur.com/ln7FGoA.png</a><br>
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<pre class="moz-signature" cols="72">--
Paul Theodoropoulos
<a class="moz-txt-link-abbreviated" href="http://www.anastrophe.com">www.anastrophe.com</a></pre>
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